Method for detecting an electroconductive element in a document

ABSTRACT

The invention relates to a method for detecting an electroconductive element in a document, a signal with a modulated carrier frequency being coupled to at least one transmitting electrode. This signal is transmitted via the elecroconductive element to at least one receiving electrode, the received modulated signal being evaluated for detecting the electroconductive element. The electroconductive element in the document constitutes a capacitive coupling element between the transmitting and receiving electrodes.

This invention relates to a method for detecting an electroconductiveelement in a document according to the preamble of claim 1.

A method of the aforementioned kind is known e.g. from DE 43 39 417 A1.In the known method, the high-frequency signal produced by an oscillatoris applied to a transmitting electrode. The transmitted signal istransmitted via the electroconducive security thread to the receivingelectrode and evaluated in an evaluating circuit. The security thread isrecognized as genuine by the evaluating circuit if amplitude and phaseposition of the transmitted signal match the input signal on thetransmitter.

A problem in detecting security threads in a document is that forexample a moist document can cause a signal due to the electrolyticconduction although the document has no security thread. Reliabledifferentiation of the signal transmitted from the transmitting to thereceiving electrode due to the metallic conduction of the securitythread from a signal caused by the electrolytic conduction in thedocument is impeded, however, by the high capacitive couplingresistances present between the transmitting and receiving electrodes.At a working frequency in the 100 kHz range these resistances can be inthe order of magnitude of megaohms for example, and are thus very muchgreater than the resistance of the metallic security thread, which canbe e.g. in the kiloohm range. Due to the high capacitive couplingresistances, the contribution of the security thread can no longer beexactly detected during measurement. This is important in order toensure reliable detection of the security thread in the document.

The invention is thus based on the problem of proposing a simple andmore reliabie method for detecting an electroconductive element in adocument.

this problem is solved according to the invention by the features statedin characterizing part of he "independent" claims.

The basic idea of the invention is that for detecting anelectroconductive element in a document a signal with a modulatedcarrier frequency is coupled to at least one transmitting electrode andtransmitted by means of the electrconductive element to at least onereceiving electrode, the received modulated signal being evaluated fordetecting the electroconductive element. The electroconductive elementacts as a capacitive coupling element between the transmitting andreceiving electrodes.

The capacitively transmitted modulated carrier frequency signal ispreferably mixed onto the modulation frequency on the receiver side. Thesignal with the modulation frequency can then be evaluated as toamplitude for detecting the electroconductive element. The signal can befiltered and amplified prior to evaluation. By reason of componentavailability and costs, a value of e.g. 455 kHz as is usual in radiotechnology is preferably selected as the modulation frequency. Formodulating the carrier frequency signal one can use amplitudemodulation, whereby the carrier frequency can be in the MHz range andthe modulation frequency in the kHz range.

By using a very high carrier frequency in comparison to the modulationfrequency one can reduce the capacitive coupling resistances between thetransmitting and receiving electrodes considerably, e.g. by a factor of10³, which permits more exact detection of the electroconcuctiveelements. Due to the low capacitive coupling resistances, which are e.g.in the kiloohm range, the inventive method is insensitive to moisture inthe documents.

Since the modulated carrier frequency is mixed with the same carrierfrequency, only a UHF oscillator is required for the transmitting andreceiving electrodes. One can use e.g. a simple LC oscillator. Thispermits the method to be carried out with low technical effort.

In a development, a plurality of transmitting and receiving electrodescan also be provided for detecting the electroconductive element in thedocument. The transmitting and receiving electrodes of an electrode pairare disposed linearly to each other. Each electrode pair corresponds toa measuring track. The electrode pairs are disposed so that themeasuring tracks do not disturb each other. The use of a plurality ofmeasuring tracks has the advantage that the electroconductive elementcan be detected reliably even with a skew of the documents to be tested.Due to the increased number of measuring tracks there is also lesslikelihood of any cracks in security threads being located exactly wherethey impair detect on. This can also reduce the sensitivity to cracks inthe security thread during measurement.

Further advantages and developments of the invention result from thedependent claims and the following description of an embodiment of theinvention with reference to the enclosed figures, in which:

FIG. 1 shows a schematic ropresentation of the arrangement of thetransmitting and receiving electrodes,

FIG. 2 shows a block diagram of the inventive solution.

FIG. 1 shows by way of example two electrode pairs each havingtransmitting electrode 30 and receiving electrode 40 for detecting anelectroconductive element in a document. Document 10 can be for examplea bank note in which metallic security thread 20 is embedded in theknown way. Electrodes 30, 40 preferably have a width essentiallymatching the width of the security thread. For example the electrodescan have a width of 1.5 mm. The transmitting and receiving electrodes ofan electrode pair can be disposed at a distance of e.g. 5 mm. Theelectrode pairs can extend over the total width of the document. Theelectrode pairs are to be disposed such that when the security thread islocated in the area of the electrodes the measured signals capacitivelycoupled over from the particular transmitting electrode to theparticular receiving electrode do not disturb each other. The number ofmeasuring tracks increased by the arrangement of a plurality ofelectrode pairs is advantageous in particular upon a skew of thedocuments to be tested. Also, with a multitrack electrode arrangementextending over the total width of the document one can detect the lengthof the security thread.

However, only one electrode pair consisting of a transmitting and areceiving electrode can of course also be sufficient for detecting theelectroconductive element in the document, in particular when nodetection of the total length of the security thread is desired. Thetransmitting and receiving electrodes should then preferably be disposedin center to the document moved past the electrodes, so that thesecurity thread can cause the signal to be coupled over from thetransmitting to the receiving electrode even with a skew of thedocument.

According to the invention, a modulated signal is coupled to thetransmitting electrode and the capacitive transmission of the modulatedsignal to the transmitting electrode measured. Transmission of themodulated signal only takes place if the electroconductive element islocated in the area of the transmitting and receiving electrodes.

FIG. 2 shows oscillator 50 for producing a carrier frequency signal,e.g. of about 220 MHz, and modulation oscillator 60 whose frequency isfixed e.g. at 455 kHz The signal from the carrier frequency oscillatoris supplied via phase shifter 90, which e.g. eliminates an undesirablephase shift obtained due to stray capacitances, to mixer 70 andmultiplied by the modulation frequency there. The modulation used is a200% amplitude modulation. The modulated carrier frequency signalobtained by mixer 70 is supplied to transmitting electrode 30 andtransmitted to receiving electrode 40 by means of the electroconductiveelement not shown here. The signal obtained at receiving electrode 40 issupplied to further mixer 80 and mixed with the same carrier frequencythere, thereby recovering the modulation frequency of 455 kHz. Themodulated carrier frequency signal can be mixed e.g. by proper-phasemultiplication of the modulated carrier frequency signal by the samecarrier frequency. The signal with the modulation frequency can besupplied to narrow-band filter 110 via preamplifier 100. The filterserves to suppress noise and permits a narrow-band gain of the measuredsignal in amplifier 120. The output signal from amplifier 120, which isproportional to the logarithm of the signal amplitude and has a bandwidth of e.g. 3 kHz, is then supplied to device 130 which evaluates theamplified measured signal e.g. as to amplitude. In order to obtain thebest possible signal yield one can conpensate away the signal fractionwhich couples over from the transmitting electrode to the receivingelectrode without an electroconductive element. This can be done forexample by feedback 140 of the complementary output of mixer 70 toreceiving electrode 40.

What is claimed is:
 1. A method for detecting an electroconductiveelement in a document, comprising the steps of:generating a modulatedcarrier frequency signal by multiplying a carrier frequency signal by amodulation frequency signal; coupling the modulated carrier frequencysignal to at least one transmitting electrode; capacitively transmittingthe coupled modulated signal to at least one receiving electrode via theelectroconductive element in the document; and evaluating the receivedmodulated signal for detecting the electroconductive element in thedocument.
 2. The method according to claim 1 further comprising the stepof:mixing the capacitively transmitted modulated signal with the samecarrier frequency signal.
 3. The method according to claim 2, furthercomprising the step of:amplifying and filtering the mixed modulatedsignal before it is evaluated; wherein the step of evaluating includesevaluating the mixed modulated signal as to amplitude.
 4. The methodaccording to claim 1, wherein the modulated carrier frequency signal ismodulated by an amplitude modulation.
 5. The method according to claim4, wherein the amplitude modulation is 200%.
 6. The method according toclaim 5, wherein the carrier frequency signal is in the MHz range andthe modulation frequency signal is in the kHz range.
 7. The methodaccording to claim 6 wherein the carrier frequency signal is about 220MHz and the modulation frequency signal is about 455 kHz.
 8. The methodaccording to claim 4, wherein the carrier frequency signal is in the MHzrange and the modulation frequency signal is in the kHz range.
 9. Themethod according to claim 8, wherein the carrier frequency signal isabout 220 MHz and the modulation frequency signal is about 455 kHz. 10.An apparatus for detecting an electroconductive element in a document,comprising:a first oscillator (50) arranged to produce a high-frequencysignal; at least one transmitting electrode (30) arranged tocapacitively transmit a signal coupled thereto; at least one receivingelectrode (40) disposed linearly with respect to the at least onetransmitting electrode (30); a second oscillator (60) arranged toproduce a modulated signal; a mixer (70) arranged to receive thehigh-frequency signal from the first oscillator (50) and the modulatedsignal from the second oscillator (60) and arranged to output amodulated carrier frequency signal which is coupled to the at least onetransmitting electrode (30).
 11. The apparatus of claim 10, wherein aplurality of transmitting and receiving electrodes (30, 40) areprovided.
 12. The apparatus according to claim 10, further comprising:aphase shifter (90) arranged to supply the high-frequency signal from thefirst oscillator (50) to the mixer (70).
 13. The apparatus according toclaim 12, further comprising:a second mixer (80) arranged to mix themodulated carrier frequency signal, once it has been transmitted to thereceiving electrode (40), with the same high-frequency signal.
 14. Theapparatus according to claim 13, further comprising:a pre-amplifier(100) arranged to amplify the mixed signal from the second mixer (80); anarrow-band filter (110) arranged to filter the amplified signal; anamplifier (120) arranged to amplify the filtered signal and to producean output signal; and a device (130) arranged to evaluate the outputsignal from the amplifier (120).